According to the latest Gallup polls, public opinion on issues of sexual and reproductive freedom has become steadily more liberal. In 1977, Americans were evenly split on whether gay sex should be legal. Now they support its legality by a 2-to-1 margin. In 1996, the country opposed same-sex marriage by 68 to 27 percent. Now it’s a dead heat. In 2002, a 50-to-45-percent plurality said it was morally wrong to have a baby outside of marriage. Now a 54-to-42-percent majority says it’s acceptable. Birth control, as an issue of private morality, is a nonissue: 89 percent of Americans say it’s OK.

Will Saletan writes about politics, science, technology, and other stuff for Slate. He’s the author of Bearing Right.

On issue after issue, the polls have moved to the left. But not on abortion. In 1995, Gallup found that 56 percent of Americans identified themselves as pro-choice, while 33 percent identified themselves as pro-life. That gap closed within three years, zigzagged a bit, and by last year stood at 49 to 45 percent, a narrow pro-choice plurality. In this month’s poll, however, 50 percent of respondents call themselves pro-life. Only 41 percent call themselves pro-choice.

But that’s the puzzle. At best, support for abortion is barely holding its ground, way below support for contraception, while approval of gay sex and gay marriage are soaring. Something about abortion continues to alienate people who are willing to take a more liberal view of birth control and homosexuality. What is it?

Pro-choice groups see abortion as an issue of women’s rights, reproductive freedom, and respecting privacy. But look at long-term data from the General Social Survey, a multidecade project of the National Opinion Research Center. The survey shows that over the past 40 years, public opinion has shifted in the pro-choice direction on all three of those themes. And yet, contrary to the pro-choice inference, it hasn’t shifted on abortion.

Polls don’t settle what’s right and wrong. But they do challenge our assumptions about the structure of beliefs. When public opinion turns toward gay marriage without abandoning fidelity and family formation, it calls into question our fear that extending marriage to same-sex couples threatens the institution. And when public opinion turns toward reproductive freedom and equal rights for women but continues to oppose abortion, it punctures our dismissal of pro-life sentiment as a vestige of right-wing sexism. Spin and soundbites won’t make the evidence go away. Sooner or later, you'll have to face it.